In many fields of activity, such as surveying, construction and navigation, accurate measurement of short and medium size distances, generally no more than 100 meters is needed. The required accuracy will vary with the activity. In some activities such as global surveying and navigation, distance measurement that is accurate to within a fraction of a millimeter (mm) is required for purposes of initial positioning.
Several non-contact distance measurement schemes have been proposed by workers in this field. One approach is frequency modulation/phase detection, which can be used over distances as large as 100 meters with a distance uncertainty, about 3,000 parts per million over the full measurement range. The accuracy obtainable is often limited by phase shift due to the dynamic range of the signals received. The full measurement range is often limited to 1 meter in order to obtain measurements accurate to within 1 mm. Schiek et al, in U.S. Pat. No. 4,238,795, disclose a microwave range measuring device in which microwaves are emitted at a first frequency, then at a second frequency, and are reflected from the object whose distance is to be determined. The reflected waves return to and are sensed by the device to determined the distance. A medium distance measuring device, disclosed by Budde in U.S. Pat. No. 4,829,305, also uses return of reflected microwaves from a target to determine the distance from the device to the target.
Optical triangulation of position is another attractive approach, possibly allowing distance measurements that are accurate to within a fraction of a millimeter over a modest distance range. This approach requires emission of a collimated beam from a reference position and determination of the distance by use of geometrical calculations based upon measurements of angles of reflection of the beam at the object. Sawabe et al, in U.S. Pat. No. 4,908,648, disclose use of radiation transmitted toward and reflected by the target, using two radiation signals that can overlap in space or in time. Resolution is very much dependent upon the distance measured and becomes monotonically worse as the distance increases. Further, the emitted beam would need to be modulated to insure immunity from changes in ambient conditions.
What is needed is apparatus for distance measurements that is good to within a millimeter or a fraction thereof, that does not require use of complex, movement-sensitive optical and electronics components, that provides approximately the same accuracy over the entire measurement range, that takes account of changing ambient temperature, that requires no recalibration at subsequent times, and that is compact and rugged enough for field work.